It is well known that vehicle exhaust gases are a cause of environmental pollution. The gaseous pollutants are commonly subdivided into four broad categories: Hydrocarbons (NC), Oxides of Nitrogen (N0x), Carbon Monoxide (C0) and Carbon Dioxide (C02). Additionally, the exhaust gases comprise very small particulates (referred to as PM10s) of solid matter which have a significant effect on air quality. In North America and Europe legislation provides limits for the mass of each type of pollutant that is emitted when the vehicle is driven over a standard drive-cycle. The standard drive cycle is intended to be broadly representative of how vehicles are actually used (see for example, the Urban Dynamometer Driving Cycle from US Federal Test Procedure 72).
The emissions testing procedure cannot be expected to characterize a vehicle's emissions under all conceivable driving conditions. The standard drive cycles have been designed to be as representative as possible whilst still being a viable basis for an emissions test. Specific legislation exists in both North America and Europe to prohibit manufacturers from calibrating their engine control systems so that a significant increase in tailpipe emissions occurs when the vehicle is operating at speeds and loads not on the standard drive-cycle. This may be desirable as increased performance can be obtained from the vehicle if emissions are deliberately degraded.
The manufacturers are allowed to degrade a vehicle's emissions in order to protect the engine or emission control equipment fitted to the engine and a specific example of this is high load enrichment on spark-ignition (SI) engines. The speeds and accelerations required by this test are easily achievable by a modern vehicle and at no point does the engine get close to full load. At full load, depending on calibration, the SI engine can be operating at an air-fuel ratio that is richer than the stoichiometric ratio (normally to protect the exhaust valves). When the engine is running rich, catalyst conversion efficiency is dramatically reduced and HC and CO emissions increase considerably. Additionally, there are defined windows for each gear change on the drive-cycle that last about two seconds. In practice a gear change can be performed quicker than this. Gear changes, especially fast ones, normally result in the engine being unable to control accurately the air-fuel ratio during these rapid transients. Inaccurate control of the air-fuel ratio results in poor catalyst conversion and consequently increased emissions of HC, NOx and C0.
FIG. 1 shows a graphical depiction of the post catalyst pollutant mass of both hydrocarbons (Line A) and N0x (Line B) as the air-fuel ratio (AFR) is varied. For fuel rich AFRs the HC emissions rise sharply and the N0x emissions are low. For fuel lean AFRs, the N0x emissions rise and the HC emissions are low. When there is a stoichiometric AFR then the N0x and HC emissions are equal and at a relatively low level.
Compression-ignition (CI) engines are capable of running at a wide range of air-fuel ratios. In a CI engine, the air-fuel ratio is varied in order to vary the torque output of an engine. SI engines use a throttle to restrict the mass of air inducted into the engine to achieve the same torque reduction effect. The emissions of HC, N0x and C0 are related to the air-fuel ratio and injection timing being used for a CI engine. Richer mixtures tend to result in lower temperature and incomplete combustion, resulting in increased HC and CO emissions.
Injection timing also has an effect on the level of emissions. A CI engine has an optimum injection angle for efficiency, although emissions considerations may force the controller to deviate from the optimum. Injection timing affects the peak temperature achieved during combustion. At high combustion temperatures, atmospheric nitrogen is fixated and N0x emissions arise. Other factors, such as instantaneous catalyst conversion efficiency, the use of exhaust gas recirculation (EGR), time since start and particulate trap state also affect tailpipe emissions on SI and/or CI engines. Considering this range of factors, it can be seen that there are many modes of driving which generate more pollutants than the figures predicted by standard drive cycles.
Further to the standards for vehicle emissions over a defined drive cycle, the engine control system on a vehicle must also monitor the performance of emissions control equipment. If a fault is detected in the emissions control equipment that could result in an increase in tailpipe emissions, the engine controller warns the driver by illuminating a “check engine” lamp on the instrument cluster. This lamp is referred to as the “malfunction indicator lamp” and the driver is expected to take the vehicle for service if the lamp becomes illuminated. In order to detect these faults, the engine controller contains a suite of diagnostics (OBD) software that monitors engine performance. The OBD standard also specifies a protocol that allows proprietary software tools to interrogate the engine controller. This interface allows access to fault codes that are stored inside the engine controller. OBD must also support the reporting of real-time measurements made by the engine controller, such as engine speed, calculated load, etc.
As part of the homologation process for a new vehicle, it will be subjected to an emissions test, during which a driver will be required to control the vehicle's speed to a set point as determined by the drive cycle. Exhaust gases from the vehicle are stored in a bag which is subdivided into a number cells, which allows a small gas sample to be collected once a second on the drive cycle, At the end of the test, the gas samples are analyzed to determine the mass of HC, N0R, CO and C02 in each sample. The equipment used to perform the gas analysis is bulky (usually one wall of a large room) and this technology is not suitable for on-vehicle processing of emissions.
Alternative measurement techniques are now available: Fast NO and HC sensors have been developed (for example by Cambustion in the UK) and allow instantaneous measurement of pollutant mass. This equipment is expensive and still relies on bottled reference gases, rendering this technology unsuitable for use for on-vehicle emissions testing. FastN0 sensors, suitable for on-vehicle use, are in development for advanced Diesel emissions control systems but this technology is not yet mature. An equivalent HC sensor is not currently available and the cost of retro-fitting these sensors to a vehicle and interfacing them to the emissions control systems will still be high.
A known technique is disclosed by U.S. Pat. No. 6,604,033, in which a system is provided that uses exhaust gas sensors and data provided by an onboard diagnostic system to determine the emissions of a vehicle and whether or not they meet a regulatory threshold. The most significant disadvantage of the system disclosed in U.S. Pat. No. 6,604,033 is that the exhaust gas sensors are expensive and will need to be installed to each vehicle for which the emissions are to be measured.